The water soluble blood group substances, A, B, O(H), Le super a and Le super b, are genetically determined glycoproteins with highly specific serological reactions. Essentially, these oligosaccharide structures are present on the erythrocyte surface and are responsible for the specificity of the antigens on the red blood cells. The objective is to change the serological specificity of the A and B blood and make available a Universal Donor Type. This can be done by the use of enzymes involving specific degradation (glycosidases) or biosynthetic pathways (glycosyl transferases). This entails the study of immunochemistry and genetics of blood group substances, purification of enzymes involved and testing the effect of these enzymes on the physiological integrity and viability of erythrocytes.